Summary / Abstract To orchestrate complex processes like motility and endocytosis, cells rely on actin filament regulatory proteins to control both the dynamics and architectures of actin filament networks. Actin filament nucleators are critical actin regulators, because they allow cells to initiate new actin networks, but they also dictate the architectures of these networks. Arp2/3 complex is a seven subunit 250 kD actin nucleating machine that specifically generates branched actin filaments. Regulated branching by Arp2/3 complex is required to assemble branched actin networks important for processes such as endocytosis and motility of growth cones, and in pathogenic processes like host cell infection by bacteria or metastasis of tumor cells. The majority of studies on Arp2/3 complex regulation have focused on WASP proteins, the best studied NPF. These studies revealed the basic tenets of WASP mediated activation; that it binds to the complex, recruits actin monomers, and stimulates an activating conformational change. However, a critical facet of WASP-mediated activation of the complex remains unknown: how do WASP proteins activate Arp2/3 complex to create exclusively branched actin filaments? Based on the efforts of several labs, we now know that in addition to WASP and WASP- recruited monomers, Arp2/3 complex must also bind preformed actin filaments to permit WASP-mediated activation. Despite the essential role for preformed filaments in ensuring a branch, almost nothing is known about how filaments contribute to activation. Further, it is not known how WASP, WASP-recruited monomers, and actin filaments coordinately activate branching nucleation with strict bimodal switch-like behavior, in which all activating factors are required to turn on the complex. Here we use a series of biochemical, structural, and computational approaches to address these issues. We anticipate that deciphering the molecular mechanism or WASP-regulated branching will help us understand how cells control the dynamics and architectures of the actin cytoskeleton.